The present invention relates to an automatic machine for washing articles, for instance laundry, in a bath containing surfactant substances.
In the case, for instance, of a laundry washing machine, a machine of this type is usually provided with an electromechanical or electronic programming and timing unit, and may also have manually operable means for presetting the various parameters for a washing process to be carried out, such as the amount and type of laundry, the degree of dirtiness, temperature and filling level of the washing solution, etc. All of these devices are associated with the operative components of the machine and permit the user to select a considerable number of washing programs to be carried out by the machine depending on the particular characteristics of the laundry to be cleaned. Each such program may include various phases such as a pre-washing phase, main washing phase, rinsing phase, and centrifuging or spinning phase. Some of these phases may be repeated once or several times for ensuring the best result. After any such program has been suitably selected, it is carried out by the machine in an automatic manner and is terminated after a preselected time which may be different for any given program, the duration of each program having been calculated by the designer of the machine on the basis of laboratory testing procedures.
It is obvious, and in practice it is generally the case, that the operating conditions of the machine in practical use rarely correspond to those assumed by the designer. As a result, any washing cycle may be terminated before the laundry is completely clean, while in other cases the duration of the washing cycle may be longer than actually required. In the first case the result of the washing operation will be unsatisfactory for the user, while in the second case there occurs a waste of energy, water, and detergents.
Various systems have already been devised for avoiding the shortcomings outlined above, but none of these proposals has provided a fully satisfactory solution due to the fact that it is not possible to directly and reliably determine the degree of dirt removal from the laundry.
Known, for instance from British Provisional Patent No. 1,258,839, is the use of a photosensitive device for monitoring the degree of transparency of the water during the rinsing phases and for causing the washing cycle to be terminated when the rinsing water has attained a predetermined degree of clarity. This device cannot, however, ensure complete cleaning of the laundry, as the rinsing water may attain the selected degree of clarity even if certain amounts of dirt still adhere to the laundry. On the other hand, this device does not allow the required amount of detergents, water and energy to be reduced to the indispensable minimum for complete removal of the dirt from the laundry. In other words, this solution only permits a saving of water in the rinsing phases.
From German DE-OS No. 28 54 148, for example, it is also known to employ a sensor disposed within the washing tub for measuring the specific conductivity of the water at successive intervals. The thus determined values are stored and compared to one another. When the difference between two successive values is smaller than a preselected reference value, the rinsing phase is terminated. A similar system is described in French Patent Application No. 2,485,576, employing the use of luminous devices for continually monitoring the degree of purity of the water during the washing and rinsing phases.
From British Patent No. 2,052,251 it is known to control the operation of a laundry washing machine by measuring the surface tension, the hardness, the electric conductivity and the pH value of the water for suitably metering the amounts of water and detergents required for the washing process.
None of the above listed solutions appears fully satisfactory, mainly due to the fact that they are based on the determination of factors which are not truly indicative of the progress of the laundering operation. As a matter of fact, the transparency, the electric conductivity, the hardness and the acidity of the washing bath are scarcely indicative of the actual removal of dirt from the articles to be cleaned. The surface tension, although to a certain degree indicative of the detergent effect of the surfactant substances contained in the detergents supplied to the washing bath, also does not represent totally accurate information. As a matter of fact, the surface tension may be influenced by other variables such as the hardness of the water and the temperature.